This invention relates to a method and apparatus for determining noninvasively volumetric flow rate of fluid flowing through a conduit and, more particularly, of flow through a blood vessel. The term "noninvasive" refers to the fact that the invention can be practiced without surgically exposing any portion of the body or even injecting dyes into the bloodstream, which provides obvious advantages over blood flow measuring techniques which cannot be practiced noninvasively.
Volumetric blood flow to major body organs is an important physiological parameter for proper diagnosis of many diseases. Insufficient blood flow to many organs can result in damage and ultimate necrosis. Further, insufficient blood flow to the heart and brain can result in coronary heart disease and stroke, two of the major causes of death in the United States today.
One of the major causes of reduced blood flow is an obstruction in a blood vessel caused by arteriosclerosis. Reduced blood flow is also caused by inadequate blood pumping of the heart or low blood supply in the body. Since volumetric blood flow is an indication of the amount of oxygen being delivered to various regions of the body, its determination can assist the physician in his management of disease and trauma. Thus, the quantitative measurement of blood flow has widespread application in the diagnosis of vascular and related diseases, evaluation of the effects of therapeutic treatment on blood flow, and postoperative evaluation of vascular surgery.
Many techniques for measuring blood flow such as the electromagnetic flow meter are regarded as unsatisfactory since they require implantation through surgery. Other techniques such as the use of angiograms which provide an X-ray image to show obstructions in the vessel have been used, but they are also unsatisfactory since a radio opaque dye must be injected into the vessel which involves some hazard and discomfort to the patient and dynamic information such as blood flow rate cannot be determined.
Ultrasonic flow meters have been considered the most promising approach for noninvasive blood flow measurement. When an acoustic beam of ultrasonic wave energy is radiated into the body, the wave is backscattered by moving particles, such as red cells in the bloodstream, which cause the frequency of the wave to be shifted by an amount proportional to the blood particle velocity, a phenomenon known as the Doppler effect. However, altough relative blood velocity information has been determined by such prior art ultrasonic flow meters in measuring the Doppler frequency shift, the actual velocity of blood flowing through a vessel has not been determined.